This invention relates to a paper machine fabric capable of being formed with a wide range of porosities and yet retains a constant yarn count and caliper. More particularly, the porosity and density of the paper support surface remains constant which allows for a controlled high fiber retention while the total fabric porosity is variable between a preselected range as desired.
In the past attempts have been made to form paper machine fabrics having controllable porosity without recognizing the desirability for a constant yarn count and fabric caliper. For example, U.S. Pat. No. 4,379,735 is concerned with lateral or weft wise stiffness of a paper machine fabric. The patent discloses forming vertical drainage channels and varying the porosity by varying the fabric mesh or count along with varying the warp and weft yarn thickness. U.S. Pat. No. 4,867,206 is concerned with forming a two ply fabric having vertical drainage channels in each ply which interconnect to form vertical drainage channels which get progressively larger toward the lower surface. U.S. Pat. No. 4,640,741 provides a multi-layer paper machine fabric having three layers of vertically stacked weft yarns and two layers of warp yarns forming vertical drainage channels. The lower layer has a "coarser weft density" than that of the intermediate layer and the weft density of the intermediate layer is smaller than the uppermost weft layer. The porosity is controlled by varying the warp density and yarn count. U.S. Pat. No. 4,941,514 is concerned with providing a paper machine fabric in which vertical drainage channels are formed by vertically stacking three layers of weft yarns. The patent discloses varying the fabric permeability by varying the thickness of the intermediate layer of weft yarns. Finally, U.S. Pat. No. 4,909,284 is directed to a paper machine fabric in which a support surface is formed with two sets of weft yarns, a lower surface is formed with one set of weft yarns. The lower weft yarns are vertically aligned with one set of the upper weft yarns.
None of the above patents recognize the problem of maintaining a support surface of constant count and density, varying the density of the intermediate layer while maintaining a constant count, and maintaining the lower layer constant while at the same time maintaining a constant fabric caliper.
It is a further object of this invention to provide a constant paper support surface so that markings formed are minimal and uniform and the paper fiber retention remains constant.
It is a further object of this invention to provide a constant fabric caliper so that the pressure exerted on the paper fibers is uniform.
It is a further object of this invention to vary the porosity of the paper forming fabric between selected limits while maintaining a constant yarn count.
It is another object of the invention to provide a paper machine fabric which has been heat set under tension for stability, has a constant caliper, and which is of controlled permeability while the paper fiber support surface does not vary.